Add a missing file to Android.mk

Change-Id: I05e3cddb2ed44d7d77272dc4293e961c24fad2f8

jni/Android.mk

diff --git a/jni/Android.mk b/jni/Android.mk
index 6cffaae..20b484d 100644
--- a/jni/Android.mk
+++ b/jni/Android.mk
@@ -185,6 +185,7 @@
                    $(CERES_SRC_PATH)/schur_jacobi_preconditioner.cc \
                    $(CERES_SRC_PATH)/schur_templates.cc \
                    $(CERES_SRC_PATH)/scratch_evaluate_preparer.cc \
+                   $(CERES_SRC_PATH)/single_linkage_clustering.cc \
                    $(CERES_SRC_PATH)/solver.cc \
                    $(CERES_SRC_PATH)/solver_utils.cc \
                    $(CERES_SRC_PATH)/sparse_cholesky.cc \